Wood Quality, as used by the pulp and paper industries, refers to a series of wood components that, at the end of the processing, affect cellulose yield. The most studied wood components affecting wood quality include lignin content, the proportion of monolignols siryngil and guaiacyl (S/G), the module of elasticity, spiral grain, fiber characteristics and wood density. T. Markussen et al., Wood Density Silvae Genetica 53: 45-50 (2004). Of these components, lignin content and wood density significantly impact cellulose yield.
Lignin Content
Lignin is one of the major products of the general phenylpropanoid pathway, and it is one of the most abundant organic molecules in the biosphere. Lignin accounts for 20-30% of the dry weight of trees and through a process called lignification, lignin is deposited in the cell walls of supporting and conductive tissues, thereby providing rigidity to the wood and structural integrity to tracheary elements. Baucher et al., Crit. Rev. Biochem. Mol. Biol. 38: 305-50 (2003). Lignification also occurs following microbial infection or wounding and protects tissues from pathogen penetration. Baucher et al., (2003); Boerjan et al., Annu. Rev. Plant. Biol. 54: 519-46 (2003); Crawford, LIGNIN BIODEGRADATION AND TRANSFORMATION, New York: John Wiley and Sons (1981).
Lignin's resistance to degradation significantly limits the use of lignocellulosic materials, as lignin must be removed during pulping and papermaking and this requires environmentally hazardous chemicals. Baucher et al., (2003); Whetten et al., Forest Ecol. Management 43: 301 (1991). Current research efforts aim to develop trees with reduced lignin content, thereby reducing the amount of chemicals needed for kraft pulping. Baucher et al., (2003); Sederoff et al., GENETIC ENGINEERING OF PLANT SECONDARY METABOLISM, New York, Plenum Press (1994).
Lignins are the result of dehydrogenative polymerization of monolignols, notably p-coumaryl, coniferyl and synapyl alcohols. Reviewed in Boejan et al., Annu. Rev. Plant. Biol. 54: 519-46 (2003). Different plant species or cell types harbor lignin polymers composed of varying proportions of these three monolignols. For example, gymnosperm lignin is primarily composed of guaiacyl (coniferyl-derived) units, whereas angiosperm lignin is primarily composed of guaiacyl and syringyl (synapyl-derived) units. Grass lignin, on the other hand, is a mixture of guaiacyl, syringyl and p-hydroxylphenyl (coumaryl-derived) units. Campbell and Sederoff, Plant. Physiol. 110: 3-13 (1996). It is well known that the monomeric composition of lignin has a significant effect on its chemical degradation during industrial pulping. Baucher et al., Plant Physiol. 112: 1479-1490 (1996); O'Connell et al., Transgenic Res. 11: 495-503 (2002); Baucher et al., Critical Reviews in Biochemistry and Molecular Biology 38: 305-50 (2003).
Several steps in the monolignol biosynthesis pathway, leading to lignin synthesis, represent SAM-dependent methylation reactions, evidencing the importance of this methionine-derived substrate in lignin biosynthesis. SAM is synthesized from methionine by the action of one or more SAM synthetase isoforms and is used as a cofactor in many processes in plant cells besides lignification, such as DNA methylation and ethylene, biotin and polyamine biosynthesis. Ravanel et al., Proc. Natl. Acad. Sci. USA 95: 7805-7812 (1998).
Wood Density
In the assessment of raw-material quality for pulping, wood density is another important parameter. Wood density significantly influences the yield and quality of fibrous and solid wood products, as well as strength, machinability, conversion, wearability, and paper yield. Bamber and Burley, The wood properties of Radiata Pine. Commonwealth Agricultural Bureau. Slough, p. 84 (1983). High wood densities are advantageous because they correspond to higher pulp yields on a raw-material volume basis, and to a better use of digestor capacity. From the vantage point of forest production, high wood density combined with high volume growth maximizes production on the unit area basis. Miranda et al., Forest Ecology and Management 149: 235-40 (2001).
While wood density is a critical factor in the profitability of kraft pulp production, increasing wood density is difficult because it is a complex trait that is not easily managed for breeding purposes. For temperate softwood, the average ring density depends on the earlywood and latewood proportion and the relative densities of each of them. Thus, improved wood density is the result of various combinations of components that could be changed by manipulating one or more components. Louz, Ann. For. Sci. 60: 285-94 (2003). Further adding to the difficulties associated with increasing wood density, hardwood tree improvement is a slow process because of the lengthy time needed for breeding a single generation. This process is made even more difficult by the changes that occur during the transition from juvenility to maturity.
Although the wood density is a complex trait, it presents great variations between trees as well as high heritability and reduced Genotype×Environment interactions. In analyzing wood densities in different species, it has been shown that wood density has a strong genetic component. Louzada and Fonseca, Ann. For. Sci. 59: 867-73 (2002).
There is consensus amongst the forest breeding community that if one could develop methods that allow early selection at the individual level, this would be of considerable value for increasing the genetic gain per unit time. In this regard, current research efforts are focused on identifying molecular markers that cosegregate with complex traits, such as wood basic density. Grattapaglia et al., Genetics 144: 1205-14 (1996). Most of these studies have associated quantitative trait loci (QTLs) with juvenile wood density. For example, four markers showed a consistent association with juvenile wood density in segregating populations of Pinus radiata. Devey et al., Theor Appl Genet 108: 516-24 (2004).
Accordingly, there is a continuing need to identify genes whose expression can be affected to increase wood density and decrease lignin in plants, in particular, woody tree species.